The present invention relates to high input/output (I/O), high density, low cost electronic modules and, more particularly, to the high I/O, high density, low cost packaging of high performance, high density semiconductors having impedance-controlled transmission line buses and, optionally, driver line terminators built into the modules, for maintaining high electrical performance.
A current trend in electronic package design for high speed, high performance electronic systems is to provide one or more multi-wiring layer, multi-chip ceramic or polymeric composite modules to provide the high performance, high density and highly reliability interconnections needed between the various circuit devices that form functional systems or parts of larger systems. The system may be a mainframe computer, a workstation, a telecommunications network, or any other electronic equipment.
Today""s ceramic and polymeric composite modules are available in a wide variety of sizes and complexities ranging from small single-chip modules with a few layers of wiring to multi-chip modules greater than 100 mm square supporting over 100 chips and containing up to 90 layers of wiring. These modules predominantly use pin-and-socket or pin-grid-array (PGA) connectors for electrical interconnection to the motherboard. Both plated-through-hole (PTH) and surface mount technology (SMT) sockets are available. While both versions present an electrical discontinuity, the PTH version is electrically worse due to the extra pin length. However the SMT version, which typically has the sockets mechanically connected as part of a molded housing, has an important limitation: due to differences in the coefficient of thermal expansion (CTE) between the socket and the motherboard. This CTE difference limits the maximum number of solder joints for interconnection on the bottom of the module to between 400 and 500. For even higher quantities of interconnections, far more expensive solutions are required. An example of this is the Harcon(copyright) connector system on IBM""s large thermally cooled modules (TCM""s). In one recent example 4224 pins are included on a 127 mm square module. Even so, since the pins are unshielded, they present a significant electrical discontinuity to the system in today""s high speed systems. One such system operates at a speed of 630 megahertz. Also, the maximum density achievable with an array of Harcon connectors is a 2.2 mm by 2.4 mm interstitial grid. Therefore the present ceramic and polymeric composite modules lack at least one of the following:
a) cost effective, high quantity I/O interconnections or connections to the motherboard
b) superior interconnection or connection electrical performance.
High reliability for such a module, including the interconnections, is essential due to potential end product failure, should vital misconnections of these devices occur. It is also very important that both the module and especially the interconnections be as dense as possible, use the least possible amount of real estate on the module, provide high electrical integrity, and provide minimal impact on the wiring of the module as well as the mating motherboard or system board. In today""s highly competitive marketplace, both the initial as well as the long-term cost of the module are also very important.
As system density and performance have increased so dramatically, so have the stringent specifications for interconnections. One way high electrical performance is manifested is in improved signal integrity. This can be accomplished by providing the interconnections with shielding that helps them to more closely match a desired system impedance. These demanding requirements, especially when coupled with the requirement for field-separability, have led to a wide variety of possible connector solutions.
Also, to assure effective repair, upgrade, and/or replacement of various components of the module (e.g., connectors, cards, chips, etc.), it is desirable that the connections within the module be reworkable at the factory. It is also highly desirable in some cases that, within the final product, such connections be separable and reconnectable in the field. Such a capability is also desirable during manufacturing in order to facilitate testing, for example.
A land grid array (LGA) is an example of such a connection in which each of two primarily parallel circuit elements to be connected has a plurality of contact points, arranged in a linear or two-dimensional array. An array of interconnection elements, known as an interposer, is placed between the two arrays to be connected, and provides the electrical connection between the contact points or pads. For even higher density interconnections and savings in the real estate of the motherboard, additional parallel circuit elements may be stacked and electrically connected through additional LGA connectors to create three-dimensional packages. In the prior art, a stacked structure relies on ball grid array (BGA) solder joints for the abovementioned connections. In any case, since a retentive force is not inherent as in a pin-and-socket type interconnection, a clamping mechanism is needed for LGA connections to create the force necessary to ensure each contact member is compressed an appropriate amount during engagement to form the required interconnections to the circuit elements. While LGA interposers and clamps are implemented in many different ways, the implementations of most interest are those described in the aforementioned copending U.S. patent applications.
For some applications between two primarily parallel circuit elements, the connections can be BGA solder joints instead of the LGA interposer described above. The BGA solder joints can be used in combination with the interposer. For example, within the stack of a stacked module, the connections can be BGA solder joints while the connections between the module and the motherboard can be provided by an LGA interposer. The use of BGA solder joints, however, will eliminate the convenience of field separability.
Alternatively, the LGA interposer can be replaced by a conventional PGA connector for module to motherboard connection while the LGA interposers are used within the module. The use of a PGA connector for connection from module to motherboard is important for those applications where the new module will be used to replace or upgrade an existing module (i.e., where the interconnection is already defined). An example of this is a microprocessor socket on a personal computer motherboard.
One of the most important factors for data moving into and throughout the module is that the effective impedance of the signal propagation paths is well controlled, and one end of the bus is terminated, preferably on the module, to the characteristic impedance of the system in order to maintain signal fidelity and signal integrity. Any impedance mismatches along the signal transmission path result in signal degradation which, in turn, may lead to errors in data transmission. Because of their design, conventional LGA connectors introduce excessive impedance mismatches and crosstalk that degrade signal quality and therefore limit the performance of the module signal channels and are less desirable for use both within the module and for connection to the motherboard.
It is therefore an object of the invention to provide a high I/O, high density, high reliability module for high performance semiconductors.
It is an additional object of the invention to provide a high I/O, high density module utilizing a novel high density connector technology and is transparent to a number of connector technologies.
It is another object of the invention to provide a high I/O, density impedance control led module that may be field demountable and upgradable.
It is a still further object of the invention to provide a high I/O, high density module that is a cost effective replacement of the current multi-chip modules used in high speed electronic systems.
The present invention is a cost effective module that provides high performance, high I/O, high density and highly reliability interconnections needed between the various circuit devices that form a functional system or a part of a larger system. It includes a plurality of circuit members, some having contact pads and circuit devices on their surfaces, also including impedance-controlled transmission line signal paths to support high speed operation. Short land grid array connectors or interchangeable alternative connectors provide interconnections between the circuit members and the rest of the system. Driver line terminators may be included on the circuit members for maintaining high electrical performance. The circuit members may be formed on conventional printed circuit cards including integrated and shielded spacers, with unpacked or packed semiconductor chips and other components attached directly to the circuit members. Clamping means is also included. Thermal control structures may be included to maintain the circuit devices within a reliable range of operating temperatures. Suitable applications include but are not limited to mainframe computers, workstations, telecommunications networks, or other electronic equipment.